1. Field of the Invention
The present invention relates to a digital radio communication system in the form of a multi-points to single point time division multiplex network.
The present invention is advantageously used for a high speed and high frequency band digital subscriber radio communication system, for instance, a so-called local area network (LAN).
2. Description of the Related Art
Recently, demands for a high speed and high density communication system have greatly increased in large scale office automation. As one known example for satisfying these demands, there is a high frequency band (for example, 26 GHz) digital subscriber radio system for the LAN.
This system comprises a central station and a plurality of substations (i.e., subscribers). The multi-points to single point time division multiplex network is comprised of these stations. A central station is used in common by these substations and each substation is arbitrarily added to the network. The central station, in general, comprises one working and one standby transmitter and receiver unit. The standby transmitter and receiver unit is designed to deal with a failure of the working transmitter and receiver unit, and has the same structure as that of the working transmitter and receiver.
In the central station, however, problems arise regarding high speed digital signal propagation. That are, when the working transmitter and receiver is changed to the standby transmitter and receiver, a phase in a sampling clock signal and a difference in signal propagation delay time for the transmission signal occur between the working and the standby units. This phase lag and delay time are caused by, first, differences in length of the connection cables between the working and the standby units, and, second, differences in electrical characteristics between the working and the standby units. As a result, signal propagation delay time of the transmission signal occurs between the central station and each substation.
Although the phases of the sampling clock signal and the propagation delay time are adjusted between the working transmitter amd receiver unit and the standby transmitter and receiver unit, it is troublesome to perform the work needed to make the adjustment of the phase and delay time since the transmitter and receiver are usually located at the top of a radio tower, and conversely, radio line terminating equipment is located at the bottom of the radio tower.